This invention relates to a variable draft vessel having twin hulls for use with a drilling platform and other deck load and which is adapted for deep water drilling and other operations.
In attempts to locate new oil fields, an increasing and significant quantity of well drilling has been conducted in offshore sea and like locations where a substantial body of water overlies the oil field. This has generated considerable interest and effort in offshore and deep water drilling. One current method of offshore drilling utilizes a fixed drill platform mounted on legs resting on or driven into the sea floor. These, however, are feasible for use only in relatively shallow depths of water normally not greater than about 300 feet, which is a realistic depth limit for practical commercial operations. Deep water drilling has heretofore been accomplished with the employment of specifically designed and constructed vessels or rigs which have certain inherent disadvantages and limitations noted hereinafter. A brief review of both offshore and deep water drilling methods heretofore practiced and the vessels or rigs employed therewith will provide a more clear appreciation and understanding of the present invention, as well as a clear distinction between those vessels or rigs employed in offshore drilling and those employed in deep water drilling.
An early method of offshore drilling, still currently employed, provides for the erection of a self-contained fixed platform which is supported by pilings driven into the sea floor and has a drilling rig, auxiliary equipment and crew's quarters located on the platform. At the conclusion of the drilling, a tender is brought to dismantle and remove the drilling equipment and, in the case of a dry hole, the entire self-contained platform is dismantled and removed by tender. A variation of the foregoing method provides a somewhat smaller platform similarly erected on piles and having a drilling rig located thereon while the auxiliaries, equipment and crew are located in a tender tied alongside. At the conclusion of the drilling, the platform is likewise either left for oil production or is dismantled and removed in the case of a dry hole. Another method employs a self-elevating barge which is towed to the drilling site and provided with columns or legs which are then lowered and embedded in the sea floor. The barge is then jacked up so as to clear the water surface and serves as a platform on which the drilling rig, crew, and auxiliary equipment are subsequently positioned for drilling operation. At the conclusion of the drilling, in the event of a producing well, a fixed platform generally is erected for continuing oil production and the self-elevating barge is towed to another drilling site. The foregoing methods are each feasible for use in relatively shallow water depths of normally 300 feet or less. The factors governing construction and operation of the vessels or rigs utilizing any of the foregoing arrangements are not significantly concerned with stability and other problems involved in conducting a drilling operation from a floating platform, as in deep water drilling, since the above-discussed rigs and vessels are constructed for sea bottom engagement.
Deep water and exploratory drilling has been accomplished by means of surface floating drilling vessels which are either towed or self-propelled to the drilling site and are self-contained in that the drilling rig, auxiliary equipment and crew's quarters form an integral part of the vessel. These floating drilling vessels are anchored over the drilling site and are normally provided with a central opening through which the drilling rig is operated. Drilling operations from these floating vessels are, however, highly restricted by sea state conditions, since excessive vessel motion in heave, pitch and roll can and does damage the drilling equipment as well as aggravate the problem of maintaining the vessel anchored directly over the drilling site. The stability characteristics of such a single-hulled drilling vessel are accordingly not conducive to efficient oil drilling operations. A catamaran type oil drilling vessel has been constructed; however, while that type vessel affords greater stability than a single-hulled vessel, it involves substantial problems which include excessive vessel motion due to wave action, no substantial motion minimizing characteristics, overstability and resultant "snapping" action which tosses personnel about and may endanger the drilling string and other equipment. Accordingly, while these vessels are not geographically limited to offshore drilling operations, they are limited to use in restricted or calm waters.
Deep water and exploratory drilling operations have also employed semisubmersible platforms which, like the floating vessels, are completely self-contained. In this latter type, the platform is supported on a plurality of structural members including stabilizing columns joined at their lower ends to a base flotation structure which, when in unballasted condition, floats the entire structure above the surface of the water with the base structure having freeboard. After being towed in freeboard condition to the drilling site, the base flotation structure is deballasted to submerge the same. To maintain the drilling platform above water in a relatively stabilized condition, the platform support structure includes a plurality of columns which extend between the platform and the base structure and are partially submerged to the extent that the displacement of such columns in conjunction with any residual displacement of the base structure supports the overall semisubmerged structure.
In one type of these prior semisubmersible platforms, the stabilizing patterns of the stabilizing columns form substantially symmetrical equilateral polygons with the columns located at the apices of the respective polygons which are normally square or triangular in shape. The equilateral symmetrical polygonal arrangement provides substantially identical righting moments about the roll and pitch axes, as well as any intermediate axes, regardless of wave direction. Another form of similar semisubmersible vessel includes a plurality of stabilizing columns interconnected adjacent their upper ends so that only the columns float in the water, with the columns again being arranged in a symmetrical equilateral polygon. In general, while such prior vessels of this type provide adequate stability for well drilling operations, they have an inherent disadvantage of very low mobility between drilling sites due to the shape and frontal area of the polygonally arranged columns and/or base structure presented to the water surface when such vessels are towed; their towing speed often does not exceed 2 knots. Additionally, to achieve the necessary displacement for supporting the full weight of drilling rig, deck load, etc., for such a vessel having a symmetrical equilateral polygonal configuration of base structure and stabilizing columns, such prior type vessels are of such large dimension that they cannot pass through narrow waterways like the Panama and Suez Canals, whereby their use is accordingly restricted.
Accordingly, it is a primary object of this invention to provide a new improved twin hull variable draft vessel which minimizes above-discussed and other shortcomings of prior offshore and deep water drilling rigs and vessels, and provides various advantages in construction, mode of operation and result over prior systems.
It is another object of the present invention to provide a variable draft vessel which, particularly in column stabilized floating condition, has the characteristic of minimizing vessel motion due to excitation forces caused by wave action (hereafter called "motion minimizing characteristic"). It is a related object to provide such a vessel affording improved motion minimizing characteristics in vessel pitch, roll and heave.
It is a related object of the present invention to provide a variable draft column stabilized vessel having rapid mobility in transit.
It is still another object of the present invention to provide a twin hull variable draft column stabilized vessel which is operable in great depths of water as a self-contained drilling vessel.
It is a further object of the present invention to provide a variable draft vessel having twin hulls which has a drilling platform mounted on a deck which is above the hulls an effective height slightly greater than the maximum anticipated wave height.
It is still a further object of the present invention to provide a variable draft drilling vessel having stabilizing columns configured to reduce displacement of the vessel in heave in the high draft floating condition due to the inertia thereof and to provide reduced pitch and roll angles.
It is still a further object of the present invention to provide a deep water drilling vessel having twin hulls the draft of which is variable by ballasting and deballasting of the twin hulls to selectively orient the vessel in a low draft floating condition with the hulls having freeboard and a high draft column stabilized floating condition.
It is a related object of the present invention to provide a variable draft deep water drilling vessel having twin hulls including separate ballast compartments in each hull together with ballasting means for selectively ballasting and deballasting the several compartments in each hull thereby selectively correcting the heel and trim angles.
It is another object of the present invention to provide an improved variable draft vessel usable as a tender for other vessels, or as a platform for a heavy duty crane, dredge or other equipment, as well as for a drill rig.
These and other related objects and advantages of the present invention will become more apparent from the following specification, claims, and appended drawings wherein: